Method of and means for operating electric furnaces



June 9, 1925.

C. STEENSTRUP METHOD OF AND Imus FOR OPERATING ELECTRIC FURNACES W M A-H:@ m A- Filed llay l 1. 1923 Patented June 9, 1925.

UNITED STATES onms'rmn STEENSTRUP, or SCHENECTADY, NEW YORK, ASSIGNOR T0GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK.

METHOD OF AND MEANS FOR OPERATING ELECTRIC FURNACES.

Application filed May 11,

To all whom it may concern:

Be it known that I, CHRISTIAN STEEN- STRUP, a citizen of the UnitedStates, residing in Schenectady, county of Schenectady, State of NewYork, have invented certain new and useful Improvements in Methods ofand Means for Operating Electric Furnaces, of which the following is aspecification.

In the operation of electric furnaces of the resistor type, especiallythose of the closed type in which the heating operation is carried outin the presence of a selected gas, as hydrogen for example, it isnecessary to vary the amount of current which passes through theresistors during the cycle of operation. For example, when the furnaceand its contents are cold a greater amount of current is utilized thanwhen the parts are heated to substantially the operating temperature. Ithas been the common practice to use resistance external to the furevenvery nace for cutting down or regulating the current supplied to thefurnace which is of course highly wasteful of electric energy.Furthermore, these resistances which are costly to make and occupyconsiderable.

valuable space, are frequently burned out in practice which means asubstantial loss in money and alsoa loss due to interruption of thefurnace operations. This latter is particularly serious where thefurnace is used in the commercial production of some product. It may beso serious as to necessitate shutting down a department which isdependent on the furnace while repairs are being made.

In the manufacture of resistors it is very diflicult, if not impossible,to make the material used in the construction so uniform incross-section and in resistance that the temperature at every point inthe resistor is exactly the same as that at every other point. If thecross-section at one point is slightly smaller than that at theremainder, that point will get hotter than it should and is therefore apotential danger spot because of the liability to (rupture. Furthermore,it is necessary to support the resistors at suitable points byinsulators of some character which either partly or.

wholly enclose the resistor at the points of support. As a result ofthis, the radiation of heat from the resistor is impaired more or lessat said points which means that 1923. Serial No. 638,343.

the resistors get hotter at said points than in regions between them.

As is well known, if a piece of metal is highly heated at one spot, asby a gas flame, when the flame is removed, the heat at the hot spot willbe rapidly conducted by the metal itself to the cooler portions. Thisfact is utilized in the operation of the furnace aswill appear morefully later on.

In carrying out my invention, I dispense with all external resistance inthe supply conductors and thereby obviate the troubles incident to theiruse. This is accomplished by using contactors orswitches which directlymake and break the main line or supply circuits to the resistors and bycontrolling the contactors through magnetic means which are responsiveto suitable make and break devices. Instead of continuously supplyingcurrent to the resistors and varying the amount required to obtain thedesired furnace temperature by the wasteful resistance method, thecurrent is supplied intermittently, the periods when it is flowingdepending on the temperature desired inside of the furnace. When thefurnace and its contents are relatively cold the periods of admissionare greater than when they are relatively hot. The nearer the furnace isto the operating temperature the shorter will be the periods ofadmission and the greater will be the care that must be exercised in itsoperation.

I have found from actual experience that a higher average amount ofcurrent can be admitted to the resistors by this method than wheretheflow of current is continuous. This is due to the fact that the hotspots in the metal are permitted to equalize with the cooler portionsduring the periods that the current is cut off.

In order to determine the temperature of the resistors within thefurnace various means may be employed but by preference ordinary sighttubes are provided and arranged so that theoperator can see theresistors and observe'their appearance. By this means it can readily bedetermined if the resistors are too hot or too cold or if some point orpoints are showing signs of being overheated. My experience has furtherdemonstrated that these observations and changes in current flow can beeasily made by workmen of ordinary intelligence.

In the accompanying drawing is diagrammatically illustrated suitableapparatus for practicing my invention.

1 indicates an electrical furnace of any suitable construction, and 2the suitably placed sight tubes for observing the conditions insidethereof and specifically of the resistors. The resistors?) may be madeof any suitable material and can be arranged in any suitable way. In thepresent instance the resistors are-made of iron. In practice theresistors are commonly located along the top and sides of the furnaceand the work is placed in the center of the furnace chamber andsupported by a suitable bed of heat insulation. In all, three resistorsare shown, but a greater or lesser number may be provided. They arearranged to be connected in multiple directly across the supplyconductors without the inclusion of any external resistance. Eachresistor is so arranged that its supply of current can be separatelyregulated if desired or all the resisters may receive the same amount ofcurrent for the same periods of time. In the supply conductor 4 of eachresistor is a contactor or switch 5 which comprises a movable and astationary contact. I have not illustrated the specific construction ofthe contacts for these are well known in the art. All the resistors areconnected to a common return conductor 6. The movable contact of theswitch is actuated by an electromagnet 7 although other forms of relaymechanism to accomplish the same purpose may be employed if desired.Each magnet receives current from a suitable source of supply through arevolving contact drum 8. On each drum are one or more V-shaped contacts9 and a body of insulation 10. Engaging with the drum is a contactfinger or brush 11 which is connected to a solenoid or other magnet. Thecontacts 9 are connected to the supply wire 12 through the brush 13 andthe opposite ends of the magnets to the return wire 14. The brushes aremounted so as to be movable along the drum in an axial direction wherebythe portion of each revolution that the brush is on a V-shaped contactcan be varied to energize the assoeiated magnet for a greater or lesserperiod. For this purpose the brushes are mounted on adjustable leadscrews 15. The drums are mounted on a spindle 16 which is driven by anelectric motor 17 through worm or other gearing 18. The motor may run atconstant speed or its speed may be changed by ordinary regulating means.In the present embodiment of the invention the drum revolves twice perminute.

Assuming that the furnace and its contents are cold or relatively so,the brushes 11 are so adjusted as to bear on the contact plates 9throughout each revolution or at least a large portion thereof. Thisresults in energizing the magnets for corresponding periods of timeduring which the contacts 5 are closed thus controlling the amount ofcurrent admitted to the resistors. As the furnace temperature increasesthe period in each revolution of the drum that the eontactors is closedis reduced by moving the brushes toward the left. The position of thebrushes is adjusted right or left as the case may be until the desiredamount of current is periodically admitted to and cut off from theresistors. To determine this the operator makes observations through thesight tubes from time to time. When the operating temperature of thefurnace is attained, the brushes 11 will usually be in engagement withthe contacts 9 for less than half of the time. The contacts 11 may makeand break the relay circuit once for each revolution of the drum or twoor more times per revolution. This will be governed by the operatingconditions and by the revolutions per minute of the drum. If aninspection shows that one resistor is hotter or cooler than itshould bethe brush l1 controlling its contactor dependently of the otheis to makethe period of current flow longer or shorter as desired. It sometimeshappens that it is desired to operate one resistor at a temperature different from the others which temperature a may be higher or lower thanthe others. The independent adjustment of the brushes ll readily permitsof this.

My invention is useful in connection with electric furnaces intended forvarious purposes but is especially useful in connection with electricfurnaces designed to unite various articles by the fusion of metal, asfor example, fusing steel parts with copper. Such furnaces commonlyoperate at temperatures from 1200 C. to 1300 C. By adjusting the brushes11 with respect to the drum contacts and driving the drums at suitablespeeds the desired temperatures of the resistors can be maintainedwithin close limits.

Another and very important advantage of my invention is that I am ableto use cheap iron units for the resistors instead of the very expensivemolybdenum units. This is due to the very close regulation obtainablewhen operating the furnace in accordance with the method hereindescribed. An iron unit operates safely at about 1500 C. which while itis considerably lower than the operating temperature of a molybdenumunit is substantially above the operating temperature of the furnace. Bymaking suitable observations of the conditions in side of the furnacefrom time to time and making proper adjustments of the regulatingmechanism, the desired temperature of the furnace may be easilymaintained which temperature is below the safe working temperature ofthe iron resistors. The saving can be adjusted inin the initial cost ofiron units over molybdenum is very great. In a certain large furnace now0 )erating in accordance with my invention t e material in the molbdenum units which were formerly emp oyed cost about $1500, whereas thecost of the material for the iron units now employed is only about$3.00.

In the operation of hydrogen furnaces the charge often comprises partswhich differ greatly in mass; this is particularly true when it iscomposed of turbine parts such as a rotor having a massive centerportion and relatively small buckets and a cover that are to be copperbrazed to the periphery of the center part. In such cases it isimportant to control one or more of the heatin units independent] of theother or ot efs. For example, the heating unit adjacent the small artsshould not receive the same amount 0 current as the heater adjacent themore massive parts because they will be overheated. It has been proposedto employ tem erature indicators at one or two places wit in the heatingchamber and to automatically regulate the supply of current to theheating units as determined by the indicators. Such an arrangement isentirely satisfactory for many classes of work but for copper brazingoperations I find that the automatic control is not satisfactoryprincipally for the reason that the indicators only indicate thetemperatures in relatively small regions of the heating chamber with theresult that some parts of the charge are either over or under heated andhence the best results are not obtained. With my improved arrangement,however, the operator first observes the conditions within the furnaceand then adjusts the relative positions of one or more brushes and theircontact-controlling drums to the end that the average amount of currentsupplied to any given unit or units' is exactly rlght to produce thedesired heating effect. In some instances more than a single observationand adjustment may be desirable. In other words the heating effect ofone unit can be increased or decreased with respect to another asexperience indicates should be. done and this without necessarilyinvolving any appreciable change in the temperature of the heatingchamber as a whole.

I have particularl referred to controlling the temperature 0 the furnaceand its char e pre aratory to the brazing operation, but it is a sodesirable in some cases, as for exam le where the center part is massiveand t e outer part thin, to control the temperature of the parts duringthe cooling operation. That is to say, under the conditions specified,current may be admitted to the heating unit adjacent said thin outerpart while the suppl of current to the eating unit adjacent t e massivepart is cut oil, so as to obtain a more uniform cooling of the charge asa whole.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. The method of operating an electric furnace which comprises'perlodically admitting heating current to and cutting it off from theheating unit thereof and in varying said periods.

2. The method of operating an electric furnace which comprisesconnecting the heating unit directly across the supply conductorsthrough a switch, opening and closing the switch to intermittentlysupply current to the unit, and varyin the eriods that said switch isopened an closed.

3. The method of operating an electric furnace having independentheating units which comprises directly connecting the heating units tosupply conductors through independent relay actuated switches, andperiodically operating said relays to open and close the switches.

4. In combination, an electric furnace, a heating unit therefor, supplyconductors for the unit, a switch for connectin the unit directly acrossthe conductors, re ay mechanism for operating the switch, positivelyacting means for operating the mechanism periodically, and a device forchanging the periods of operation of said means.

5. The method of operating a closed furnace in which a supply ofselected gas is maintained and which is heated by a resistor whichcomprises directly connecting the resistor across the supply means anddis connecting it therefrom at regular and predetermined periods, andvarying the relative periods that the resistor is in circuit.

6. The method of operating an electric furnace having independentheating units which comprises directly connecting the units across thesupply conductors through independent switches, openin and closing theswitches periodically, an varying the eriodic action of said switcheseither inependently or simultaneously, to suit the temperatureconditions of the different parts of the charge within the furnace.

In witness whereof, I have hereunto set my hand this 10th day of May,1923.

CHRISTIAN 'STEENSTRUP.

